The present invention relates to a vehicle driveline master clutch which utilizes a ball ramp mechanism to load a clutch pack and more specifically to a vehicle driveline master clutch which utilizes a ball ramp mechanism to load a clutch pack where the relative rotation of the ball ramp mechanism within the clutch is controlled through indexing plates to provide proper clutch actuation irregardless of the direction of torque flow through the driveline.
Driveline master clutches commonly use a plurality of springs to clamp one or more friction discs to an engine flywheel, where the clamping springs are normally disposed within a pressure plate assembly which is bolted to the flywheel. The friction discs are splined to rotate with a transmission input shaft which, when rotated and clamped to the flywheel provide motive power to the driveline and wheels of the vehicle. A mechanical linkage operated by a driver/operator is used to control the engagement and disengagement of the master clutch.
Efforts to automate the operation of the master clutch to eliminate the need for driver intervention are currently underway. Thus, it is known to make use of a hydraulic actuator or an electric motor actuator to operate the master clutch release mechanism in response to a control signal generated by a control microprocessor which processes a multiplicity of sensor outputs which are used to determine the vehicle operating conditions and hence the desired operation of the master clutch. Furthermore, the use of a ball ramp actuator to operate a driveline master clutch is known in the art. U.S. Pat. Nos. 5,441,137; 5,469,948; 5,505,285; 5,651,437; 5,810,141; 5,910,061; 5,964,330; and RE 36,502 assigned to the same assignee as this application, all of which are hereby expressly incorporated by reference, disclose methods of using a ball ramp actuator to supply the clamping force on a clutch disc and could, in the alternative, be used to supply a release force against a clutch apply spring.
Typically, a ball ramp actuator is activated when an electrical current is supplied to a coil thereby producing an electromagnetic field in a coil pole which applies a retarding force to an armature which rotates with an input shaft. The rotating armature is nonrotatably connected to an annular control ring which has a plurality of control ramps or grooves formed in the face of a control ring which vary in axial depth. An annular activation ring which rotates with an output shaft has a like number of variable depth activation grooves formed therein which oppose those formed in the control ring where a corresponding number of rolling elements are trapped between the control and activation grooves. As a retarding force is applied to the control plate by the armature, the rotational movement of the control ring relative to the activation ring causes the rolling elements to simultaneously traverse the control grooves and the activation grooves thereby causing an increase in separation distance between the control and activation rings which is used to provide a clamping force on a clutch friction disc.
Also shown in the prior art are other types of vehicle driveline devices which make use of a ball ramp mechanism to provide a clamping load on a clutch pack. U.S. Pat No. 5,092,825 discloses a limited slip differential having a clutch pack loaded by a ball ramp actuator. U.S. Pat. No. 5,499,951 discloses a driveline transfer case where the torque split is controlled by a ball ramp actuator. U.S. Pat. No. 5,528,950 discloses a transmission inertia brake where a ball ramp actuator loads a clutch pack to slow the rotation of a spinning transmission shaft. U. S. Pat. No. 5,819,883 discloses a driveline retarder in which a ball ramp actuator is used to load a clutch pack to rotate a hydraulic pump in response to a signal from a control unit. The disclosures of U.S. Pat. Nos. 5,092,825; 5,499,951; 5,528,950 and 5,819,883 are all hereby incorporated herein by reference.
The prior art ball ramp actuators used in the operation of the master clutch or other driveline coupling system such as a differential or transfer case could be improved by improving the inherent mechanical stability of the ball ramp. It would also be an advantage if the master clutch remained fully engaged irregardless of the direction of the flow of torque through the clutch. Unidirectional grooves in the ball ramp mechanism could provide for increased separation distance capability since the axial depth variation in the grooves can be made greater because twice as much length is available as compared to a bi-directional groove. However, prior art ball ramp actuated driveline clutches cannot provide continuous engagement and increasing clamping force as the torque through the clutch is reversed, such as when a vehicle transitions from drive to coast modes.
The present invention results in an improvement in the operational characteristics of a ball ramp actuator which can be used in a variety of vehicle driveline applications to provide a clamping load on a frictional clutch pack. The present invention provides for the same unidirectional apply ball ramp mechanism functionality as disclosed in U.S. Ser. No. 09/940,821, assigned to the same assignee as this application, without the use of one-way clutches. The present invention makes use of separate index plates to control the direction of rotation of both the control ring and the activation ring functioning similar to the one-way clutches which they replace. In this manner, the reliability of the clutch is improved while manufacturing costs are reduced.
As taught in the U.S. Ser. No. 09/940,821 application, one-way clutches can be used to control the rotational direction of the control ring and control plate where the control plate can also function as a coil armature and the rotational direction of the activation ring and activation plate to provide a continuous clutch apply function using unilateral grooves in the control ring and activation ring where the clutch pack clamping force is maintained irregardless of the direction of the flow of torque through the clutch assembly. However, one-way clutches are expensive and not as reliable as the other clutch components and the use of the index plates of the present invention in place of the one-way clutches results in a cost reduction with improved reliability.
The present invention provides the basic function of the one-way clutches using index plates which function to limit the degree of rotation of the control ring and the activation ring. The rotation limitation provided by the index plates is not identical to that provided with the one-way clutches, but for this many applications of a ball ramp mechanism, such as where it used to apply a clamping load to a clutch pack in a driveline master clutch, the functionality of the index plates is satisfactory. Specifically, a one-way clutch will lock-up during counter rotation after a relatively small degree of rotation (on the order of one degree) while the index plates of the present invention can allow as much as 240 degrees of rotation prior to lock-up depending on travel required. When the rolling elements of the ball ramp mechanism are at the bottom of their respective ramps, both index plates are against their respective stops formed in the control ring and the activation ring so that any differential rotation between the output shaft and the input shaft will result in an increase in separation distance in the ball ramp mechanism to increase the clamp load on the clutch pack.
Thus, the present invention improves the performance and durability of a ball ramp actuated master clutch assembly by eliminating the need for one-way clutches. Indexing plates are used to limit the relative rotational travel of the control ring and the actuation ring of the ball ramp actuator which is used to apply an axial clamping load on a clutch pack. One indexing plate limits the rotation of the control ring of the ball ramp mechanism and a second indexing plate limits the rotation of the activation ring. Using the indexing plates of the present invention, the ball ramp mechanism, when energized, can only further compress the clutch pack with any degree of clutch slippage, thereby preventing any break in clutch engagement when the torque flow in the driveline reverses direction from a drive mode into a driven mode. The torque flow in the driveline is in a drive mode when the engine is supplying power to the input shaft of the clutch assembly and in a driven mode when the engine is absorbing power from the input shaft of the clutch assembly. The indexing plates do not immediately prevent rotation in an undesired direction as with the one-way clutches disclosed in U.S. Ser. No. 09/940,821 but permit limited rotation until the index plates hit against respective stops formed in the control ring and the activation ring.
To activate the ball ramp actuator an electrical current is applied to a coil assembly which generates an electromagnetic force that frictionally rotationally couples a control plate (or armature), an intermediate plate and an activation plate together. The control plate rotates with the control ring and the activation plate rotates with the activation ring. Slippage is allowed to occur between the activation plate and the intermediate plate or between the control plate and the intermediate plate as required to increase the activation level in the ball ramp mechanism. The index plates are oriented to only allow the control ring to rotate in an opposite direction from that of the activation ring. An activation plate rotates with the activation ring and is supported on the input shaft while the intermediate plate is driven by the clutch hub while the coil armature rotates with the control ring which is supported on the input shaft
The present invention also results in an improvement in the operational characteristics of a ball ramp actuator which can be used in a variety of vehicle driveline applications to supply a clamping load to a frictional clutch pack. The present,invention provides a unidirectional apply ball ramp function which applies the clutch irregardless of torque flow through the clutch assembly along with significantly increased frictional damping in the ball ramp mechanism itself to control and stabilize the ball ramp mechanism thereby improving the operation of the clutch or other driveline device. The operation of the ball ramp mechanism is improved by significantly increasing the frictional damping using an intermediate plate disposed between the activation plate and the control plate where the intermediate plate rotates with the output shaft and output hub. Note that the torque flow from the input shaft to the output shaft can be reversed so that the torque flows from the output shaft to the input shaft and the ball ramp clutch assembly will continue to provide the desired functionality.
One provision of the present invention is to provide a ball ramp actuator to load a clutch pack where the clutch clamping load is maintained irregardless of the direction of the flow of torque through the clutch assembly.
Another provision of the present invention is to provide a ball ramp actuator to load a clutch pack where the clutch clamping load is maintained irregardless of the direction of the flow of torque through the clutch assembly using index plates in the ball ramp actuator.
Another provision of the present invention is to provide a ball ramp actuator to load a clutch pack where the frictional damping of the ball ramp mechanism is substantially increased to improve operation.
Still another provision of the present invention is to provide a ball ramp actuator to load a driveline master clutch having improved operational characteristics.